1. Field of the Invention
The present invention relates to a steering control method, a steering control device, and a watercraft including an electric steering mechanism, for example, a steer-by-wire type steering mechanism.
2. Description of the Related Art
FIGS. 11 through 14C show a steering control method related to the present invention.
A steering wheel and a steering device, such as an outboard motor main body, are electrically connected together in a watercraft including an electric steering mechanism. As shown in FIG. 11, steering operation is controlled in a manner such that a target steering angle of the steering device is set in accordance with a steering wheel rotational angle.
More specifically, a steering wheel rotational angle θh is computed based on a detection signal of a steering wheel rotational angle sensor (step S41). A steering angle ratio K corresponding to a watercraft speed is set (step S42). The steering wheel rotational angle θh is multiplied by the steering angle ratio K, and a target steering angle θs* (=K·θh) is obtained (step S43). The steering device is instructed to make a steering operation based on the target steering angle θs* (step S44). The steering device operates in a manner such that an actual steering angle θs corresponds to the target steering angle θs* (step S45).
The steering angle ratio K is a ratio of the target steering angle θs* to the steering wheel rotational angle θh and is a constant value depending on the watercraft speed. For example, in the case where the steering angle ratio K is 1/24, the steering device steers 15° for each rotation (a 360° rotation) of the steering wheel.
However, an inconvenience may occur with such a steering control method because the target steering angle θs* is set based on the steering wheel rotational angle θh.
First, the watercraft includes an electric steering mechanism, and thus the steering wheel can be turned to a different rotational position independently of the steering device when the power supply is turned off, for example, in safekeeping the watercraft on water, on land, and so forth. Conversely, the steering device can be turned to a different steering position independently of the steering wheel. In these cases, the actual steering angle θs may be offset from the steering wheel rotational angle θh and in turn the target steering angle θs*. This may result in a circumstance that the steering wheel and the steering device suddenly turn to prescribed positions (positions that the steering wheel rotational angle θh corresponds to the actual angle θs) as soon as the power supply is turned on and a rider of the watercraft will feel uncomfortable.
More specifically, for example, when the steering wheel has been turned and the steering wheel rotational angle θh has become θ1, even though the steering device has not been steered in a state where the power supply is turned off (a state before starting) as indicated in FIG. 12A, if the power supply is then turned on at time t1 and the watercraft is started as indicated in FIG. 12C, the steering wheel may suddenly turn so that the steering wheel rotational angle θh decreases from θ1 to θ2. Therefore, the rider of the watercraft may feel uncomfortable. Furthermore, for example, in the case where the steering device has been steered and the actual steering angle θs has become θ3 although the steering wheel has not been turned in the state where the power supply was turned off (the state before starting) as indicated in FIG. 12B, if the power supply is then turned on at time t1 and the watercraft is started as indicated in FIG. 12C, the steering device may be suddenly steered so that the actual steering angle increases from θ3 to θ4. This may cause the rider of the watercraft feel uncomfortable.
Second, in the case where the watercraft has a plurality of operating stations (for example, cockpits) and the steering wheel rotational angles θh of these operating stations differ from each other, the steering device may be suddenly steered in response to a change in the control systems immediately after the operating stations are changed, and this could cause the rider of the watercraft to feel uncomfortable.
More specifically, for example, in the case where the steering wheel rotational angle θh of a first operating station is θ5 and the steering wheel rotational angle θh of a second operating station is θ6 (which is <θ5) as indicated in FIG. 13B, if the active operating station is changed from the first operating station to the second operating station at time t2 as indicated in FIG. 13A, the target steering angle θs* could decrease in response to a decrease in the steering wheel rotational angle θh from θ5 to θ6. Therefore, the steering device may be steered suddenly since the actual steering angle decreases from θ7 to θ8 as indicated in FIG. 13C. As a result, the rider of the watercraft may feel uncomfortable.
Third, in the case where a steering angle ratio varying function (a function that the steering angle ratio is varied in accordance with watercraft speed to enhance safety during traveling) is installed in the watercraft, if the watercraft speed is changed, a steering angle corresponding to a counter-steering may suddenly change although the steering wheel is not turned. This could result in the rider feeling uncomfortable.
More specifically, for example, in the case where the watercraft is traveling at a constant speed v1, starts decelerating at time t3, attains a speed v2 at time t4 to finish decelerating, and thereafter travels at a constant speed v2 as indicated in FIG. 14C, the steering wheel rotational angle θh may be retained at a constant value θ9 through all steps of the traveling as indicated in FIG. 14A. However, the actual steering angle θs starts increasing from θ10 after a deceleration starting time t3, and the actual steering angle θs could reach θ11 at a deceleration finishing time t4 as indicated in FIG. 14B. As a result of this, the rider of the water craft may feel uncomfortable.